1. Field of the Invention
The present invention relates to a flat membrane element and a manufacturing method thereof.
2. Description of the Related Art
A flat membrane element of this kind is set in water to be processed in a dipped manner and used to filtrate the water to be processed. FIG. 5 is a partial sectional view showing a general structure of a flat membrane element. A support plate 1 is formed such that a circumferential edge 2 thereof has a flame shape and a water passing portion 3 is formed in the inner portion thereof. A membrane sheet 4 is formed on both the face of the support plate 1. A suction port 5 is connected with a circumferential edge 2. In addition, the outer surface of the circumferential edge 2 and the membrane sheet 4 has a joint portion 6 formed by lapping each other. By effecting negative pressure in the suction port 5 or by effecting positive pressure from the outer surface of the membrane sheet 4, the water to be processed is filtrated and the filtrated water transmitted through the membrane sheet 4 is discharged from the suction port 5 via the water passing portion 3. Note that the water passing portion 3 of the support plate 1 is specially treated to have a figure through which the filtrated water can pass freely while supporting the surface of the membrane sheet 4 to prevent the membrane sheet 4 attached from sagging.
The membrane sheet 4 is called for example an ultrafiltration membrane or an ultrafilter membrane, and that made of synthetic resin is used in general. Also, the support plate 1 made of synthetic resin, which shows a favorable jointing characteristic with the membrane sheet 4, is used. It is important for the flat membrane element to ensure watertightness of the joint portion 6, so that a method of jointing the support plate 1 and the membrane sheet 4 with an adhesive is widely employed. However, the method of using the adhesive easily causes deviation in adhesive force in addition to the problems that the adhesive easily deteriorates to lower the watertightness and that leak from the lateral direction along the joint surface of the membrane sheet 4 is easily caused.
As a method of improving the problem in the jointing method using the adhesive, a flat-membrane-element manufacturing method in which a membrane sheet is deposited on a support plate by lapping the membrane sheet and the support plate each other and emitting a laser to the mutually jointed portion is disclosed in Japanese Patent Application Laid-Open No. 2005-279580. In this method, the membrane sheet is made of a material having larger laser penetration and the support plate is made of a material having larger laser absorption property, in which the membrane sheet is deposited on the support plate by emitting a laser from the membrane sheet side to the joint portion while pressing the membrane sheet toward the support plate. In the method disclosed in Japanese Patent Application Laid-Open No. 2005-279580, the flat membrane element having strong adhesive force at the joint portion in which the deterioration and the leak from the lateral direction of the membrane sheet are hard to be caused can be manufactured at relatively low costs.
However, some flat membrane elements contain an inorganic material such as ceramics or metal fine particles as a membrane sheet. In the flat membrane elements as described above, even when the laser is emitted from the membrane sheet side, the laser does not penetrate due to the inorganic material interrupting it. Therefore, it is impossible to deposit the joint portion of the membrane sheet and the support plate, indicating a problem that the method disclosed in Japanese Patent Application Laid-Open No. 2005-279580 is not applicable thereto.